1. Field of the Invention
The present invention relates generally to implanted devices and methods for detecting object orientation and position.
2. General Background and State of the Art
Wireless micro-stimulators such as the BION® implants (U.S. Pat. Nos. 5,193,539, 5,193,540, 5,312,439, and 5,324,316; herein incorporated by reference) can be injected into paralyzed muscles or the stumps of amputated limbs to restore voluntary control of limb movement. In order to control neuromuscular stimulation, it is desirable to provide information about the posture and movement of the limb. Two sources of such information can be distinguished. The first derives from the fact that most patients will have residual voluntary control over some portions of the limb. Such voluntary movements can be sensed by the present invention in order to provide command information about the patient's intended limb movements. The second derives from the use of feedback information to improve motor performance. The movements produced by stimulation of the muscles can be sensed by the present invention, thereby providing feedback signals that modify the electrical stimulation pattern in order to achieve the desired limb movement. In both cases, it is desirable to employ sensors that are no more intrusive than the injectable stimulators themselves. Furthermore it is desirable to minimize the number of separate devices that must be injected into the patient while maximizing the information that is available for control of the prosthetic system.
There are many technologies for tracking and measuring limb movement in the laboratory but these generally require physically affixing sensors or markers to the surface of the limb. Most commonly, the position and/or orientation of these sensors or markers are determined by capturing optical information by video cameras or photodiode arrays whose position and line of sight must be carefully determined and maintained. Search coil systems use large coils to create homogeneous, orthogonally oriented magnetic fields around the subject, which are detected by a detection coil such as attached to a contact lens to track eye movement. Polhemus Technology (U.S. Pat. Nos. 6,624,626 and 6,400,139) and Ascension Technology (U.S. Pat. Nos. 6,528,991, 5,953,683, and 4,945,305) use externally mounted sensors consisting of three orthogonal detection coils to detect the relative strength of a magnetic field created by an external field generator. In these magnetic sensing systems, the magnetic sources can be AC (U.S. Pat. No. 6,690,963), DC (U.S. Pat. No. 4,945,305) or sourceless (U.S. Pat. No. 5,953,683 earth magnetic field). There are also mechanical transducers such as electrogoniometers produced by Infotronic Company that can be affixed across adjacent limb segments to detect angular rotation of the joint in one or two axes.
The prior art includes a fully implanted sensor of a single joint angle based on the effect of a permanent magnet on a nearby Hall-effect sensor. The magnet is implanted in the bone on one side of the joint and the Hall-effect sensor is implanted on the other side of the joint so that changes in joint angle produce a systematic change in the relative orientation of the sensor to the magnet. The sensor is connected by electrical leads to an implanted signal processor that, in turn, transmits data by wireless telemetry to an external controller.
The prior art includes a sensing system to track the tip of an intravascular catheter as it is maneuvered within a patient (U.S. Pat. No. 6,690,963). This system uses a large external array of magnetic coils to create a magnetic field that is sensed by orthogonal sensing coils contained within the tip of the catheter. In the preferred embodiments of the invention the coils have mutually orthogonal axes. The coils are closely spaced along the axis of the catheter. The magnetic field generating coils are arranged across the surface of the table on which the subject lies. The sensing coils are connected by electrical leads passing along the catheter to external signal processing instrumentation. This system basically uses the same mechanism as the concentrated orthogonal 3-axis magnetic sensor mentioned above. Sensing coils with distributed positions and orientations will present more than one local minimum points in the problem. Algorithms without global optimization technique tend to set design limitations on sensing coil arrangement such that the coils are close enough to the concentrated and orthogonal configuration.
Various algorithms have been described to transform the amplitude and phase information detected by sensors into conventional coordinate frames. Both rotation matrix and quaternion have been used to provide complete descriptions of 3-D orientations and rotation transforms. Mathematical methods exist to convert between them or combine them into hybrid representations. The data processing methods include both iterative and non-iterative techniques. U.S. Pat. No. 6,690,963 teaches numerical algorithm to solve the nonlinear equation group. But it does not investigate the local minimum points associated with the numerical process. In fact, if the receiving coils and transmitting coils are not arranged properly, there will be local minimum areas that the conventional numerical methods mentioned in this patent cannot solve. In the prior arts, the filters applied to the sensing system include mean-square filters and Kalman filters. The selection of appropriate methods for computing and describing position and orientation is an important part of the design of a sensing system that determines its practicality and performance in a given application.